The present invention relates to a system and method for maneuvering a spacecraft. In particular, the present invention relates to a system and method that carries out spacecraft maneuvering with fuel savings as compared to known systems and methods. More particularly, the present invention relates to new thruster configurations.
For a spacecraft to perform its mission generally requires control of its in-orbit position relative to the Earth. For example, the orbit of a geosynchronous communications spacecraft is controlled so that the spacecraft remains stationed above the equator at a fixed earth longitude within small tolerances in the north/south and east/west direction. This position control is necessary to allow access to the satellite using fixed-direction ground antennas and to prevent both signal and physical interference with nearby spacecraft.
The orbit control is accomplished by performing periodic orbital correction maneuvers, where thrusters are fired to change the spacecraft""s velocity. During maneuvers, the attitude of the spacecraft is controlled so that it remains correctly pointed relative to the Earth and the momentum stored by actuators such as reaction or momentum wheels is adjusted. The fuel efficiency of the system is determined not only by the thruster technology used, but also by how the attitude and momentum control and the velocity change tasks are accomplished. Systems are desired that are as efficient as possible, thereby reducing the propellant required and allowing the payload mass of the spacecraft to be increased.
Various types of thrusters may be used to produce an orbital velocity change. For example, ion thrusters, such as Hall Current Thrusters or gridded ion engines, which accelerate Xenon ions through an electric field to high velocities utilizing electric power supplied by spacecraft solar arrays to generate thrust may be employed. One drawback of ion thrusters is that when the ions impinge on spacecraft surfaces they cause damage by removing surface material. In addition, the ion thruster exhaust components can interfere with communications signals transmitted to and from the spacecraft.
Another type of thruster that may be utilized includes Arcjet thrusters that heat hydrazine decomposition products using an electric arc. Generally, in designing a spacecraft, thruster technology is selected that provides the highest propellant efficiency and is compatible with the spacecraft design.
Whichever types of thrusters are utilized, they typically are mounted directly on a spacecraft or on a boom or other structure attached to a spacecraft to provide components of thrust in at least the east/west and north/south directions.
To minimize undesirable interactions between material emanating from thrusters, whether ions or other exhaust components, and elements of a geosynchronous communications spacecraft, the ion thrusters are typically located towards the panel that faces away from the Earth, commonly referred to as the base panel. FIGS. 1a and 1b illustrate such an arrangement. Along these lines, FIGS. 1a and 1b illustrate a satellite I that includes Hall Current Thrusters 3 and 5 arranged on gimbaled platforms. FIGS. 1a and 1b depicts the thrusters oriented in a position for stationkeeping and firing to produce 45xc2x0 plume cones 7 and 9.
The thruster platforms shown in FIGS. 1a and 1b are arranged on the north and south sides of the spacecraft. This configuration is also referred to as an aft-mounted ion thruster arrangement. An advantage of this thruster arrangement is that the ions produced during thrusting may be expelled out the aft end of the spacecraft, away from sensitive surfaces and out of the path of RF signals.
Mounting ion thrusters on gimbaled platforms that permit control of the thruster vector orientations as shown in FIGS. 1a and 1b can increase fuel efficiency. The gimbaling eliminates the need to fire low-efficiency chemical thrusters such as hydrazine thrusters or bi-propellant thrusters for attitude control. Most satellites today utilize a variant of the aft-mounted arrangement shown in FIGS. 1a and 1b. 
FIGS. 2a and 2b illustrate in greater detail an aft-mounted thruster configuration arrangement referenced to the body coordinates of a geosynchronous spacecraft 11. Along these lines, FIG. 2 illustrates a spacecraft 11 having a north side 13, a south side 15, an east side 17, a west side 19, an earth facing side 21, and an anti-earth facing side 23. Solar arrays 24 and 25 extend from the north side and the south side of the spacecraft. Four thrusters 27, 29, 31, and 33 are arranged in two pairs located at the corners of the spacecraft where the anti-earth side meets the north side and the south side.
Although the thruster configuration illustrated in FIGS. 1, 2a, and 2b is prevalent in satellites today, drawbacks are associated with the arrangement, as described below in greater detail. For an earth-pointing spacecraft, the yaw axis (x) is aligned with the Zenith vector, the vector directed from the center of the Earth towards the spacecraft; the roll axis (y) is aligned with the spacecraft velocity vector, which is directed in the east direction; and the pitch axis (z) is aligned with the orbit normal, which is directed in the north direction. The angle xcex1 is the angle of the thrust vector projection in the yaw/roll plane, which is measured from the yaw axis. On the other hand, the angle xcex2 is the angle of the thrust vector from the pitch (z) axis. Typically, xcex1 is about 5 degrees to about 20 degrees and xcex2 is approximately 45 degrees as constrained by the location of the center of mass of the spacecraft, because firing a single thruster must produce near-zero torque.
The most fuel efficient and operationally simple thruster arrangement for stationkeeping would allow thrust to be applied purely in the north/south direction (along the pitch axis) and in the east/west direction (along the roll axis). Thrust would not be applied in the radial (x) direction, since thrust in this direction is of limited utility for orbit control. The north/south thrusting provides inclination vector control, and the east/west thrusting provides longitude and eccentricity control.
The aft mounted configuration does not provide the desired thrust direction de-coupling, since a large thrust component is always generated in the unwanted radial direction (along the yaw axis) regardless of which thrusters are fired. For example, firing thrusters 27 and 29 for north/south stationkeeping (inclination control) produces a thrust vector equal to 2F [cos(xcex1)sin(xcex2), 0, cos(xcex2)], where F is the nominal thruster force. Clearly, for xcex2=45 degrees and xcex1=10 degrees the x and z thrust components are nearly equal. Hence, the radial coupling for a north/south maneuver using an aft mounted configuration is approximately 100%. Although this large radial coupling does not significantly affect the fuel required to perform inclination vector control, it does introduce operational complexity since split maneuvers must be executed 12 hours apart to cancel the effects of the radial thrust.
A more serious drawback of the aft-mounted configuration is that it increases the propellant required for east/west stationkeeping. For example, when thrusters 27 and 31 are fired for east/west stationkeeping the resulting thrust is 2F [cos(xcex1)sin(xcex2), sin(xcex1)sin(xcex2), 0]. For xcex2=45 degrees and xcex1=10 degrees, the thrust vector is 2F [0.69, 0.12, 0.0]. Because longitude control can only be accomplished using the roll (y) thrust component, the fuel efficiency is reduced by a factor of 8 (=1/0.12) compared to systems that thrust directly along the roll axis (in the east/west direction). In addition, although the large radial (x) component can be used for eccentricity control, the fuel-efficiency is half that possible using thrusters that directly provide roll axis thrust. Hence, the eccentricity control fuel efficiency is reduced by roughly a factor of three (=1{square root over ((0.69/2)2+0.122))} as compared to thruster systems that can apply thrust directly along the roll axis.
Among advantages of the present invention are that it can address the above-described imitations as well as other limitations of known attitude control systems.
The present invention provides an attitude control system for a spacecraft that includes a north side, a south side, an east side, a west side, an earth facing side, and an anti-earth facing side. The attitude control system includes a plurality of gimbaled thrusters interconnected with the spacecraft. The thrusters are arranged to produce thrust in a direction parallel to a roll-pitch plane of the spacecraft.
Additionally, the present invention provides a method for maneuvering a spacecraft that includes a north side, a south side, an east side, a west side, an earth facing side, and an anti-earth facing side. The method includes gimbaling at least one of a plurality of thrusters interconnected with the spacecraft and operating the at least one thruster to produce thrust in a direction parallel to a roll-pitch plane of the spacecraft.
Still other objects and advantages of the present invention will become readily apparent by those skilled in the art from a review of the following detailed description. The detailed description shows and describes preferred embodiments of the invention, simply by way of illustration of the best mode contemplated of carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments and its several details are capable of modifications in various obvious respects, without departing from the invention. Accordingly, the drawings and description are illustrative in nature and not restrictive.